1. Field of the Invention
The present invention relates to a vital information measuring device for measuring vital information such as an arterial blood oxygen saturation or a pulse rate.
2. Description of the Related Art
There is used a pulse oximeter in the field of diagnosing a sleep apnea syndrome (SAS) (see U.S. Pat. No. 4,955,379 corresponding to Japanese Unexamined Patent Publication No. 1-153139/1989, for instance). The pulse oximeter has a measuring unit which is removably attached to a predetermined measurement site of a living body i.e. a subject. As shown in FIG. 14, red light and infrared light are alternately outputted at a relatively low sampling frequency e.g. 30 Hz, in other words, at a cycle of e.g. 1/30 sec toward the measurement site of the living body, with phases of the respective light displaced from each other. The amount of light transmitted through or reflected from the measurement site of the living body is detected, and an oxygen saturation (SpO2) in blood of the subject is measured based on the detected light amount.
There is also known a photoelectric pulse wave sensor for acquiring a photoelectric pulse waveform to assess an arteriosclerosis index i.e. a blood vessel age, or an autonomic disorder of a subject. As shown in FIG. 15, the photoelectric pulse wave sensor is adapted to acquire a photoelectric pulse waveform by outputting light of a single wavelength or white light at a relatively high sampling frequency e.g. 120 Hz i.e. at a cycle of 1/120 sec, and by detecting the outputted light. A blood vessel age i.e. an arteriosclerosis index or a like parameter can be assessed by analyzing characteristics on the photoelectric pulse waveform.
In recent years, there is a demand for a measuring device capable of obtaining a blood oxygen saturation and a photoelectric pulse waveform for use in assessment of a blood vessel age or a like parameter in pair.
Measurements of the blood oxygen saturation and the photoelectric pulse waveform are common in detecting a change in light from a living body. Accordingly, the photoelectric pulse waveform is measurable with use of the conventional pulse oximeter. However, the pulse oximeter is designed to output red light and infrared light with a sampling frequency lower than the sampling frequency used in the photoelectric pulse wave sensor. Accordingly, the photoelectric pulse waveform obtained by the conventional pulse oximeter is used at most for judging reliability of an oxygen saturation measurement value, which is varied resulting from a variation in pulse waveform due to a body movement or the like. Thus, the conventional pulse oximeter has failed to acquire a photoelectric pulse waveform for use in assessing a blood vessel age or an autonomic disorder.
In view of the above, there is proposed an idea of outputting both red light and infrared light with a sampling frequency substantially identical to the sampling frequency of an output light required in measuring a photoelectric pulse waveform for use in assessing a blood vessel age or a like parameter to obtain a blood oxygen saturation and a photoelectric pulse waveform for use in assessing a blood vessel age or an autonomic disorder in pair. With such a configuration, however, if merely acquisition of information concerning a blood oxygen saturation is required, an output operation of unnecessary light that is not used in the acquisition of the information concerning the blood oxygen saturation is conducted, which may result in waste of power consumption.